1. Field of the Invention
The present invention relates to a modulation valve device associated to a hydraulic clutch systems in industrial vehicles and others, and more particularly, to a modulation valve associated to clutch systems including various clutches for various speed ranges of the vehicles, so as to smoothly increase a hydraulic pressure to the clutch after the speed range is switched from a certain range to another range.
2. Description of the Prior Art
Generally, such a modulation valve comprises an accumulator and an orifice. The orifice is disposed in an inlet oil passage between an operation chamber of the modulation valve and a hydraulic source. When the selector valve is switched to a clutch-engaging position, the orifice restricts the flow of the operation oil into the operation chamber to prevent rapid increase of the pressure in the clutch.
It is desirable to change the control characteristics for increasing the hydraulic pressure in accordance with the selected speed range. More particularly, when the first speed range clutch is engaged, it is desirable to slowly increase the hydraulic pressure to surely prevent the shock during the engaging operation. When a high speed range (e.g., 3rd range) clutch is engaged, it is desirable to relatively rapidly increase the pressure so as to prevent a harmful slip of the clutch and a time delay before the complete engagement of the clutch.
However, the conventional structure includes only one orifice, which acts regardless of the selected range or position, so that the pressure characteristics are not changed.
Therefore, it is impossible in the conventional structures to set the pressure control characteristics suitable to all of the speed ranges. For example, if the orifice is designed to obtain the pressure control characteristics suitable to the first speed range, it is impossible to obtain the characteristics suitable to the second and third speed ranges.
Accordingly, it is an object of the present invention to provide a modulation valve device, overcoming the abovenoted disadvantages, which includes a variable orifice mechanism for selectively setting the characteristics of the orifice in accordance with the selected speed range.